1. Field of the Invention
The present invention relates to an image coding apparatus, etc., for example. More particularly, the present invention relates to a technical field in which an amount of generated code is matched with a target amount of code given to one picture without performing intra-frame feedback control.
2. Description of the Related Art
To date, in a system, etc., for transmitting bit streams of moving images, or recording the images onto a recording medium, highly-efficient coding has been performed in order to effectively use a transmission path or a recording capacity. In an image coding apparatus achieving this, a coding bit rate of the bit streams generated by an encoder is controlled to be constant to meet the transfer rate of a transmission medium. With this restriction, the amount of generated data, that is to say, the quantization step of quantization in the encoder is controlled. That is to say, for example, if an image having a complicated design pattern continues, the quantization step is determined to be large in order to reduce the amount of generated data. On the contrary, if a simple design pattern continues, the quantization step is determined to be small in order to increase the amount of generated data. Thus, the overflow or underflow of the buffer memory is prevented from occurring in order to maintain a fixed rate.
Accordingly, in an image coding apparatus according to such a related-art technique, when a complicated image continues, the step of quantization becomes large, deteriorating the image quality. Also, when a simple image continues, the step of quantization becomes small. Thus, it has been difficult to obtain an equal image quality through the overall images. In view of this problem, for example, Japanese Patent No. 3358620 has disclosed an image coding apparatus which calculates the amount of code to be allocated to each GOP (Group Of Picture) depending on the complexity of an image included in the GOP. The amount of code is calculated in such a way that if a GOP includes an image with a complicated design pattern, a large amount of code is allocated, and if a GOP includes an image with a simple design pattern, a small amount of code is allocated. The allocation is carried out in accordance with the ratio of the difficulty of coding each GOP to the sum total of the difficulties of coding a plurality of GOPs.
On the other hand, for a method of matching the amount of generated code with a target amount of code given to one picture, the step2 of TM5 (test model 5) is well used. This is a method in which the amount of code allocated to a picture is evenly divided among macro blocks (MB), the divided amount is used as a target amount of code for the MB, and feedback control is performed in the picture in order to meet the target.